The present invention generally relates to mass storage devices for use with host systems, including computers and other processing apparatuses. More particularly, this invention relates to a PCIe-based mass storage system that utilizes a hybrid drive comprising at least one high capacity hard disk component for low-frequency accessed data in a host system, along with at least one nonvolatile solid-state component for system data and intermediate storage of high-frequency accessed data in the host system.
Nonvolatile memory subsystems and mass storage devices of modern computers are typically addressed through the system bus using the southbridge or any equivalent logic, for example, an input/output (I/O) controller hub (ICH) introduced by Intel Corporation. A deviation from this scheme is the PCI express (PCIe) bus having branches originating from either node of the system core logic, including integrated system agents or un-cores embedded into modern central processing unit (CPU) dies.
PCIe has become the fast system interconnect bus of choice, and offers in its latest generation (V3.x) 1 GB/s bandwidth in each direction. Particularly the branches tied to the first node in the interconnect cascade further have ultra low latency which makes them extremely suitable for any kind of data access. Another advantage of the PCIe interconnect or any expansion slot implementation is that it offers the user a high degree of freedom with respect to populating the system with peripheral devices including nonvolatile memory devices or mass storage media.
In the past, storage-related add-on cards were typically small computer system interface (SCSI) or RAID controllers with better performance or richer feature sets than on-board host bus adapters for parallel or serial ATA devices attached via cables to the card. However, as a consequence of the miniaturization of drive technology, including weight reduction, and the introduction of solid-state media, separation and cable connectivity between the interface and the actual storage media are no longer a prerequisite. A consequent trend has been the development of new form factors, including the integration of the data carrier onto an interface card along with the control logic for a streamlined, cable-less and ultra-compact device. For a number of reasons, specifically relating to weight, space constraints and power, this type of integrated PCIe-based storage device has mostly been restricted to the use of solid-state media.
Solid-state media, particularly NAND flash memory devices, have the advantage of allowing random access of data in the array over several parallel channels, and therefore it is far superior to rotatable media with respect to access speed, I/O switching and, moreover, sustained data transfer rates. On the downside, however, NAND flash memory cannot compete with rotatable media on cost per bit, nor with respect to data retention. Specifically, whereas magnetic media such as rotatable platters have practically unlimited data retention, NAND flash cells lose data through stress-induced leakage current during normal operation, as well as simple diffusion of electrons from the floating gate through the gate oxide layer into the substrate during normal operation and when the device is powered down.
In contrast to initial expectations regarding solid-state drives (SSD) becoming a complete replacement of hard disk drives (HDD) as the mass storage media used in computers, what has emerged in practice is a functional dichotomy between solid-state drives and hard disk drives. SSDs are gaining acceptance in any function or role warranting frequent access of data. This particular scenario not only fits the strength of SSDs in delivering high I/O throughput, but further adds the benefit that, because of frequent accesses, data integrity can be easily monitored through the bit error rate of a block or page on any given read access. Any increase in the bit error rate can then be used as an indicator for creeping data deterioration through leakage current. Understandably, monitoring bit error rates requires read access of the data, which puts rarely accessed files at a greater risk of corruption than files that are frequently monitored. In so far, NAND flash memory may not be the optimal storage medium.
The other end of the spectrum focuses on archiving of rarely accessed data. The majority of this type of data tends to be multimedia files such as photographs or movie clips, or archived documents including personal data, records or even e-books that are collected after purging them from the reader of choice. This type of data may not be accessed for months or years and, while it would be possible to move them to an offline vault or burn them to optical media, it is more in line with the digital life style to have the archive available at any time, for example in the form of a centralized server.
For a simplified design of any such centralized media and document server, it would be highly advantageous to have a solution that reduces space requirements by providing a highly integrated device featuring hybrid storage technology in combination with intelligent data management to combine the best features of both solid-state and rotatable (hard disk) media with respect to access speed, I/O performance and data retention at the lowest cost per bit.